The short answer
Not usually. Moroccan food is spiced rather than hot: cumin, ginger, saffron, turmeric, paprika and cinnamon give warmth and colour, not burn. Chilli heat comes on the side as harissa, spooned onto merguez or a potato maakouda sandwich. Tagines are generally mild, and riads will tone down seasoning for children.
In more detail
The flavour of a Marrakech kitchen is warm and sweet-savoury. Lamb is cooked with prunes, chicken with preserved lemon and olives, and pastilla is dusted with icing sugar and cinnamon. Tanjia gets its depth from cumin, garlic, saffron and smen, an aged butter, cooked for hours. None of that is fiery.
If you want heat, look for harissa, the chilli paste. At the grills of Jemaa el-Fnaa, merguez goes into half a round of bread with harissa or a tomato salad, and the maakouda vendors on Rue Bani Marine serve their fried potato fritters in a sandwich with harissa. You can usually ask for it on the side or left out.
For children and anyone with a sensitive palate, Marrakech is an easy place to eat. Chicken tagine with olives and lemon, couscous, harira, brochettes and msemen with honey are all mild. Most riads will cook plain grilled chicken, rice or eggs for fussier eaters if you ask.
Ras el hanout, the spice blend sold in every souk, is the closest thing to a signature. It is fragrant rather than hot.